Ferroelectric liquid crystal display

ABSTRACT

A ferroelectric liquid crystal display having alignment films with different surface polarities. The alignment films induce an internal electric field through the liquid crystal. Because of the induced internal electric field, initial liquid crystal alignment, and subsequent liquid crystal alignments, can be performed without an externally applied electric field.

CROSS REFERENCE

This application claims the benefit of Korean Patent Application No.P2000-87052, filed Dec. 30, 2000, under 35 U.S.C. §119, the entirety ofwhich is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a liquid crystal display, and moreparticularly to a ferroelectric liquid crystal display having improvedliquid crystal alignment films.

2. Description of the Related Art

A liquid crystal display (LCD) controls the light characteristics of ascreen to display a desired image. Liquid crystals used in LCDs are in aneutral state between a liquid and a solid. That neutral state has bothfluidity and elasticity.

While there are many types of liquid crystals, one type of greatinterest is the smectic C liquid crystal. During a thermodynamic phasetransition, a smectic C liquid crystal rotates along an outer line of avirtual cone. Such a smectic C phase liquid crystal can undergo aspontaneous polarization. Such a liquid crystal is usually referred toas a “ferroelectric liquid crystal” (FLC). The FLC has been activelystudied because of its fast response time. Furthermore, FLC LCDs canhave wide viewing angles without the complications of special electrodestructures or compensating films.

There are many different FLC modes, including a deformed helix FLC, asurface stabilized FLC, an anti-FLC, a V-mode FLC and a half V-mode FLC.Hereinafter, the V-mode FLC mode and the half V-mode FLC mode will bedescribed in more detail.

FIG. 1 illustrates a liquid crystal cell having a V-mode FLC. As shown,that liquid crystal cell includes an upper substrate 1 having a commonelectrode 3 and an alignment film 5. That liquid crystal cell alsoincludes a lower substrate 11 having a TFT array 9, which includes pixelelectrodes, and an alignment film 7. A V-mode liquid crystal 13 isinterposed between the upper and lower substrates 1 and 11. Thealignment films 5 and 7 are aligned in a horizontal direction, usuallyby rubbing with a special cloth roller. The V-mode liquid crystal 13forms multiple smectic layers that have molecular structures arrangedwith desired slopes with respect to a plane perpendicular to the smecticlayers. In other words, the liquid crystal molecules have desiredinclination angles with respect to the alignment directions of thealignment films. Furthermore, adjacent smectic layers have oppositepolarities.

Light transmission through the V-mode FLC liquid crystal cell variesaccording to an applied voltage across that cell, reference FIG. 2. Theliquid crystal 13 within the V-mode FLC liquid crystal cell responds toboth positive and negative voltages. Since light transmissivity rapidlychanges in accord with applied positive and negative voltages, the lighttransmissivity verses voltage curve has a V-shape as shown in FIG. 2.Thus, light transmissivity increases regardless of polarity.

FIG. 3 illustrates a liquid crystal cell having a half V-mode FLC. Asshown, a half V-mode FLC liquid crystal 15 is interposed between anupper substrate 1 and a lower substrate 11. The half V-mode FLC liquidcrystal 15 forms multiple smectic layers in which the liquid crystalmolecules align at a desired inclination angle with respect to thealignment direction of the alignment films 5 and 7. However, as shown inFIG. 3, the liquid crystal molecules in adjacent smectic layers have thesame polarity (unlike V-mode FLC liquid crystal molecules). Such a halfV-mode FLC liquid crystal can be formed by applying a positive (or anegative) electric field to a hot liquid crystal while slowly loweringthat liquid crystal's temperature into a smectic phase.

A half V-mode FLC mode liquid crystal 15 formed in this manner respondsto only one polarity of applied voltage. Thus, as shown in FIG. 4, thelight transmissivity verse voltage curve of a liquid crystal cell havingthe half V-mode FLC has a ‘half V’ shape. Still referring to FIG. 4, asshown, the light transmissivity verse voltage curve does react,slightly, to negative applied voltages, but dramatically to positiveapplied voltages.

The thermodynamic phase transition of a half V-mode FLC liquid crystal15 is as follows:

Isotropic→nematic (N*) phase→smectic C* (Sm C*) phase→crystal

Such a thermodynamic phase transition expresses the phase of the liquidcrystal in accordance with temperature, which becomes less as phasechanges move to the right.

An isotropic phase liquid crystal 15 interposed into a liquid crystalcell aligns in parallel with the rubbing direction of the alignmentlayers when the liquid crystal temperature is slowly lowered to thenematic phase. If a sufficiently strong electric field is applied acrossthe liquid crystal cell while the liquid crystal temperature is slowlylowered more, the liquid crystal 15 is phase-changed into a smecticphase in which the direction of spontaneous polarization of the liquidcrystal molecules arranges according to the electric field in the cell.Consequently, when the liquid crystal 15 within the liquid crystal cellis subject to parallel alignment treated alignment layers, the liquidcrystal molecules arrange in a spontaneous polarization direction thatis consistent with the electric field at the phase transition, and inone of two possible molecular arrangements. As a result, the liquidcrystal 15 has a uniform alignment state.

FIG. 5 and FIG. 6 help illustrate this. First, as shown in FIG. 5, if anegative electric field E(−) is applied during alignment of the liquidcrystal 15, then the spontaneous polarization direction of the liquidcrystal 15 is along the electric field. In such an aligned liquidcrystal cell, as shown in FIG. 6, the liquid crystal arrangement ischanged by an applied positive electric field E(+), but not by anapplied negative electric field E(−).

To utilize the response characteristics of the liquid crystal 15,perpendicular polarizers are arranged on the upper and lower portions ofthe liquid crystal cell. The transmission axis of one of the polarizersis along the direction of the initial liquid crystal alignment. Assuminga liquid crystal cell having the transmission curve of FIG. 4, anapplied negative electric field E(−) does not change the liquid crystalarrangement and the perpendicular polarizer blocks light. A positiveelectric field E(+) rotates the liquid crystal alignment such that lighttransmission increases.

FIG. 7 shows an electric field applied across a half V-mode FLC liquidcrystal cell. As shown, the half V-mode FLC liquid crystal cell includesan upper substrate 1 with a common electrode 3 and an alignment film 5,a lower substrate 11 with a TFT array 9 and an alignment film 7, and aliquid crystal 15 interposed between the upper and lower substrates 1and 11. The alignment films 5 and 7 are beneficially comprised of thesame material and are subject to the same alignment treatment. Aninternal electric field, E-intra, which is contrary to the externallyapplied electric field, E-ext, depends on the alignment film materialand on the polarization of the liquid crystal 15. This internal electricfield E-intra is an induced polarization field.

As described above, the half V-mode FLC liquid crystal cell uses bothtemperature and an electric field to obtain the initial liquid crystalalignment. However, liquid crystal cells made in this manner have aproblem in that the initial liquid crystal alignment can be disturbed byexternal impacts, which almost inevitably occurs due to grinding of ashorting bar. Furthermore, simple heating of a conventional half V-modeFLC liquid crystal cell can disturb the liquid crystal alignment. Tore-establish liquid crystal alignment, both temperature and electricfield treatments are required, which is difficult to do without theshorting bar.

Therefore, a new ferroelectric liquid crystal display having a liquidcrystal alignment that can be thermally re-established would bebeneficial.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aferroelectric liquid crystal display having a liquid crystal cellalignment that can be thermally recovered if disturbed, such as byexternal impacts.

To achieve these and other objects, a ferroelectric liquid crystaldisplay according to the principles of the present invention includesdriving electrodes on an upper substrate and on a lower substrates;alignment films having different surface polarities on the drivingelectrodes; and a ferroelectric liquid crystal interposed between theupper substrate and the lower substrate.

In such a ferroelectric liquid crystal display the alignment films canhave different surface polarities produced by different materialstructures. Alternatively, if the alignment films have the same materialstructure, the different surface polarities can be produced by differentthermal treatments or by different alignment treatments.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the present invention will be apparent fromthe following detailed description with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates a conventional V-mode FLC liquid crystal cell;

FIG. 2 is a graph representing light transmissivity verses voltage ofthe liquid crystal cell illustrated in FIG. 1;

FIG. 3 illustrates a conventional half V-mode FLC liquid crystal cell;

FIG. 4 is a graph representing light transmissivity verses voltage ofthe liquid crystal cell illustrated in FIG. 3;

FIG. 5 illustrates an electric field applied to a conventional halfV-mode FLC liquid crystal cell;

FIG. 6 depicts the motion of the liquid crystal upon application of avoltage to the half V-mode FLC liquid crystal cell of FIG. 5;

FIG. 7 is a schematic view of a conventional half V-mode FLC liquidcrystal cell under an applied voltage;

FIG. 8 is a schematic view of a half V-mode FLC liquid crystal cell thatis in accord with the principles of the present invention; and

FIG. 9 is a waveform diagram of a driving voltage for the half V-modeFLC liquid crystal cell shown in FIG. 8.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring now to FIG. 8, a ferroelectric liquid crystal display (LCD)according to an embodiment of the present invention includes an uppersubstrate 21 having a common electrode 23 and an alignment film 25. Thatdisplay further includes a lower substrate 31 having a TFT array 29(including pixel electrodes) and an alignment film 27. A ferroelectricliquid crystal 33 is interposed between the upper substrate 21 and thelower substrate 31.

The alignment films 25 and 27 can be comprised of materials havingdifferent material structures or the same material structure. In eithercase the alignment films 25 and 27 are beneficially aligned by rubbing.Furthermore, the two alignment films 25 and 27 have different surfacepolarities. That is, the surface polarity of the upper substrate 21 isnot the same as the surface polarity of the lower substrate 32. Thus, aninternal electric field is induced by this surface polarity difference.

When interposed between the upper substrate 21 and the lower substrate31, the ferroelectric liquid crystal 33 has an isotropic phasetemperature. As the temperature of the ferroelectric liquid crystal 33is slowly lowered to a nematic phase, the ferroelectric liquid crystal33 aligns parallel to the rubbing direction. As the temperature isslowly lowered even more the ferroelectric liquid crystal 33phase-changes into the smectic phase by the internal electric fieldacross the liquid crystal cell. A spontaneous polarization direction ofthe ferroelectric liquid crystal 33 becomes consistent with the electricfield at the interior of the liquid crystal cell. As a result, theferroelectric liquid crystal 33 obtains a mono-stable alignment state.

To differentiate the surface polarities of the two alignment films 25and 27, the alignment film materials can be comprised of polymers havingdifferent chemical structures. For example, in the case of polyimide,the dielectric constant and the polarization can be different in thealignment layers because the alignment film materials can have differentside chain molecular substitution structures. The existence of suchsubstitution structures is well known, reference, for example, LiquidCrystal, 1999, vol. 26, no. 2, 167-169. The contents of that paper arehereby incorporated by reference.

Alternatively, if the same alignment film materials are used, differenttemperature conditions or different rubbing conditions can cause thealignment films 25 and 27 to have different surface polarities. Forexample, different surface polarities can be applied to the alignmentfilms by changing the intensity of the mechanical energy used whenrubbing the alignment films. Reference, J.App.Phys. 83(3), Feb. 1, 1999.The contents of that paper are hereby incorporated by reference.

Since the alignment films 25 and 27 have different surface polarities,an internal electric field exists within the liquid crystal cell. As aresult, an initial uniform liquid crystal alignment can be made using atemperature treatment. Thus, an external electric field is not required.Furthermore, if needed, a uniform liquid crystal alignment can bere-established using a temperature treatment without an externalelectric field being applied.

The following discussion provides a theoretical framework for aspects ofthe present invention. Further information can be found inFerroelectrics, 1988, vol. 85, page 47. The contents of that paper arehereby incorporated by reference.

The surface polarity of an alignment film is related to an interfacialenergy. More specifically, the interfacial energy corresponds to theenergy sum of dispersive components and polar components, as expressedby the following equation:

γ=γ^(p)+γ^(d)  (1)

wherein γ represents an interfacial energy of a polymer used for thealignment film adjacent the liquid crystal; and γ^(p) and γ^(d) are apolar energy and a dispersive energy, respectively.

Since the polar energy is larger than the dispersive energy, the polarenergy makes a large contribution to determining the polarity of thealignment film. As the interfacial energy of the alignment film polymerincreases, the surface polarity of the alignment film increases. As theinterfacial energy decreases, so does the surface polarity of thealignment film. Such a property can be used to differentiate thepolarities of the alignment films on the upper and lower substrates.Thus, an internal electric field can be produced by the alignment films.Accordingly, the surface polarities of the alignment films can bedifferentiated by the cleaning or rubbing energies applied to thealignment films. Thus it is possible to implement alignment films havingdifferent polarities.

FIG. 9 illustrates a driving voltage for a half V-mode FLC liquidcrystal cell that is in accord with the principles of the presentinvention. As the half V-mode FLC liquid crystal cell has an internalelectric field, the driving voltage is beneficially asymmetrical.Accordingly, a common voltage Vcom compensates for the internal electricfield of the liquid crystal cell. For example, if the actual saturationvoltage of the liquid crystal cell is approximately 3.3V, the voltagerequired for initial uniform alignment should be less than 3.3V.

The present half V-mode FLC liquid crystal cell produces an internalelectric field due to surface polarity differences of the alignmentfilm. That internal electric field enables re-establishment of a liquidcrystal alignment, which can be disturbed by aging, temperature, orexternal impact, using only a thermal (heat) treatment.

As described above, according to the present invention, alignment filmshaving different polarities are used to produce an internal electricfield. Accordingly, it becomes possible to establish an initial liquidcrystal alignment without the application of an external electric field,as well as to correct the liquid crystal alignments disturbed or brokenby external impacts.

Although the present invention has been explained with reference to anillustrated embodiment, it would be understood by ordinary skilledpersons in the art of the invention that the present invention is notlimited to that embodiment. Rather, various changes or modifications ofthe present invention are possible without departing from the spirit ofthe invention. Accordingly, the scope of the invention shall bedetermined only by the appended claims and their equivalents.

What is claimed is:
 1. A ferroelectric liquid crystal display,comprising: an upper substrate having a first alignment film with afirst surface polarity; a lower substrate having a second alignment filmwith a second surface polarity, wherein said second surface polarity isdifferent from said first surface polarity; and a ferroelectric liquidcrystal interposed between the upper substrate and the lower substrate,wherein said first surface polarity and said second surface polarityinduce an internal electric field across said ferroelectric liquidcrystal and the internal electric field establishes an initial alignmentof said ferroelectric liquid crystal.
 2. The ferroelectric liquidcrystal display according to claim 1, wherein said first alignment filmand said second alignment film are alignment treated in paralleldirections.
 3. The ferroelectric liquid crystal display according toclaim 2, wherein said ferroelectric liquid crystal is aligned inparallel to said alignment treatment direction of said first alignmentfilm and said second alignment film.
 4. The ferroelectric liquid crystaldisplay according to claim 1, wherein said first alignment film and saidsecond alignment film have different material structures.
 5. Theferroelectric liquid crystal display according to claim 4, wherein saidfirst alignment film and said second alignment film have different sidechain molecular structures.
 6. The ferroelectric liquid crystal displayaccording to claim 5, wherein said first alignment film and said secondalignment film are polyimides.
 7. The ferroelectric liquid crystaldisplay according to claim 1, wherein said first alignment film and saidsecond alignment film have the same material structure.
 8. Theferroelectric liquid crystal display according to claim 1, wherein saidfirst alignment film and said second alignment film are polymers.
 9. Theferroelectric liquid crystal display according to claim 1, wherein saidfirst alignment film and said second alignment film have undergonedifferent temperature treatments.
 10. The ferroelectric liquid crystaldisplay according to claim 9, wherein said first alignment film and saidsecond alignment film have undergone different alignment treatments. 11.The ferroelectric liquid crystal display according to claim 1, whereinsaid ferroelectric liquid crystal display is driven by an externalapplied potential, and wherein said external applied potentialcompensates for said internal electric field.